convert date and time to
extern char *tzname;
time_t *clock, char
struct tm *tm);
struct tm *tm, char
struct tm *
struct tm *
time_t *clock, struct tm
struct tm *
struct tm *
time_t *clock, struct tm
function converts a time_t, pointed to by
clock, representing the time in seconds since 00:00:00
UTC, 1970-01-01, and returns a pointer to a string of the form
Thu Nov 24 18:22:48 1986\n
Years requiring fewer than four characters are padded with leading zeroes. For years longer than four characters, the string is of the form
Thu Nov 24 18:22:48 81986\n
with five spaces before the year. These unusual formats are designed to make it less likely that older software that expects exactly 26 bytes of output will mistakenly output misleading values for out-of-range years.
function converts the calendar time pointed to by
clock to local time in exactly the same way as
ctime() and puts the string into the array pointed
to by buf (which contains at least 26 bytes) and
returns buf. Unlike
the thread-safe version
ctime_r() is not required to
gmtime() functions return pointers to
tm structures, described below.
localtime() corrects for the time zone and any time
zone adjustments (such as Daylight Saving Time in the United States). After
filling in the tm structure,
localtime() sets the
tm_isdst'th element of tzname to
a pointer to an ASCII string that's the time zone abbreviation to be used
with the return value of
converts to Coordinated Universal Time.
functions convert the calendar time pointed to by
clock into a broken-down time in exactly the same way
as their non-reentrant counterparts,
gmtime(), but instead store the result directly into
the structure pointed to by result. Unlike
localtime(), the reentrant version is not required
to set tzname.
converts a time value contained in a tm structure to a
string, as shown in the above example, and returns a pointer to the string.
uses the buffer pointed to by buf (which should
contain at least 26 bytes) and then returns buf.
converts the broken-down time, expressed as local time, in the structure
pointed to by tm into a calendar time value with the
same encoding as that of the values returned by the
function. The original values of the tm_wday and
tm_yday components of the structure are ignored, and
the original values of the other components are not restricted to their
normal ranges. (A positive or zero value for tm_isdst
mktime() to presume initially that summer
time (for example, Daylight Saving Time in the U.S.A.) respectively, is or
is not in effect for the specified time. A negative value for
tm_isdst causes the
function to attempt to divine whether summer time is in effect for the
specified time; in this case it does not use a consistent rule and may give
a different answer when later presented with the same argument.) On
successful completion, the values of the tm_wday and
tm_yday components of the structure are set
appropriately, and the other components are set to represent the specified
calendar time, but with their values forced to their normal ranges; the
final value of tm_mday is not set until
tm_mon and tm_year are
mktime() returns the specified calendar
time; if the calendar time cannot be represented, it returns -1.
is a deprecated interface that is equivalent to calling
mktime() with a negative value for
converts the broken-down time, as returned by
gmtime(), into a calendar time value with the same
encoding as that of the values returned by the
returns the difference between two calendar times,
time0), expressed in seconds.
Declarations of all the functions and externals, and the
tm structure, are in the
<time.h> header file. The
structure (of type) struct tm includes the following
int tm_sec; /* seconds (0 - 60) */ int tm_min; /* minutes (0 - 59) */ int tm_hour; /* hours (0 - 23) */ int tm_mday; /* day of month (1 - 31) */ int tm_mon; /* month of year (0 - 11) */ int tm_year; /* year - 1900 */ int tm_wday; /* day of week (Sunday = 0) */ int tm_yday; /* day of year (0 - 365) */ int tm_isdst; /* is summer time in effect? */ long tm_gmtoff; /* offset from UTC in seconds */ char *tm_zone; /* abbreviation of timezone name */
The tm_zone and
tm_gmtoff fields exist, and are filled in by
gmtime(), but are not standardized. There is no
guarantee that these fields will continue to exist in this form and they may
be altered or removed in a future release.
tm_isdst is non-zero if summer time is in effect.
tm_gmtoff is the offset (in seconds) of the time represented from UTC, with positive values indicating east of the Prime Meridian.
gmtime_r() return NULL on error. The function
mktime() returns -1 on error.
- time zone information directory
- local time zone file
- used with POSIX-style TZ's
- for UTC leap seconds
If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/share/zoneinfo/posixrules.
getenv(3), strftime(3), time(3), tzset(3), tzfile(5), zic(8)
mktime() conform to ANSI
X3.159-1989 (“ANSI C89”).
localtime_r() conform to IEEE Std
timelocal() are extensions to these standards.
ctime() function first appeared in
Version 1 AT&T UNIX.
first appeared in Version 5 AT&T UNIX,
timelocal() in SunOS 4.0.
localtime_r() have been available since
The return values of the non re-entrant functions point to static data; the data is overwritten by each call. The tm_zone field of a returned struct tm points to a static array of characters, which will also be overwritten at the next call (and by calls to tzset(3)).
ctime() behave strangely for years before 1000 or
after 9999. The 1989 and 1999 editions of the C Standard say that years from
-99 through 999 are converted without extra spaces, but this conflicts with
longstanding tradition and with this implementation. Traditional
implementations of these two functions are restricted to years in the range
1900 through 2099. To avoid this portability mess, new programs should use
The default system time zone may be set by running
zic -l timezone” as the
Avoid using out-of-range values with
mktime() when setting up lunch with promptness
sticklers in Riyadh.